1. Field of the Invention
The invention relates to airborne and embarked watch and air-surface fire control radar antennae placed in a radome.
2. Description of the Prior Art
Sea surveillance and air-sea missile fire control radars have their antennae installed in a radome, situated under the fuselage of the aircraft when it is airborne. This arrangement provides a 360.degree. range of surveillance. For reasons of cost and simplicity of the aircraft, and in particular of aeroplanes, it is desirable for the radome to be not retractable but installed in a fixed position under the fuselage.
Thus, the dimensions of the radome are defined by the amount of extra admissible aerodynamic drag caused by the radome and by the space available between the lower part of the fuselage and the ground, when landing. Once these dimensions have been fixed, the radome and consequently the radars must have the largest antenna possible in the available volume of the radome.
A surveillance radar performs a predetermined movement with respect to its sight line, which is the straight line joining the center of the antenna to the target. Two articulation axes are then sufficient in the mechanical system for orientating the antenna.
On the other hand, a fire control radar permanently measures, in a horizontal plane, the angular error between the target and the missile. The measurement must be independent of the movements of the aircraft and consequently the antenna must be stabilized along three axes. A third mechanical axis is then required so as to be able to maintain the line of sight oriented on the missile whatever the movements of the aircraft. In fact, so that, after firing, the aircraft may execute an evasive turn while continuing to guide the missile, the antenna must be stabilized during rolling so as to keep the plane of measurement horizontal. To sum up, a vertical axis allows a movement of rotation for a 360.degree. sweep and for countering the yaw movements of the aircraft. Rotation about the transverse horizontal axis compensates for the pitching movements. Similarly, a movement of rotation about a longitudinal horizontal axis compensates for a rolling movement of the aircraft.
The radar antennae already existing include a parabolic reflector and a monopulse source of energy, better known under the name "rear feed". In such a system, the source is the main active element, the parabolic element only acting as reflector, both for transmission and for reception. The source is fixed with respect to the reflector. On the other hand, this latter undergoes the rotational movement about the transverse axis and the rotational movement about the longitudinal axis. In general, the shape of the reflector is related to a strip cut out from a paraboloid of revolution and placed vertically in the radome. The last movement of rotation about the longitudinal axis means that a considerable space must be provided so that the reflector can perform this third movement. With the overall dimensions of the radome determined, the size of the reflector is considerably reduced, and consequently the range of detection of the radar is also reduced.
In other words, for a land or sea patrol aircraft with a belly radome of given size, the need in fire control mode to stabilize the antenna in the direction of the target in the space of the radome considerably reduces the size of the antenna because of the rotational movements about the longitudinal axis.
The aim of the invention is to overcome this drawback by providing an antenna with sufficient cross section and allowing fire control to be performed while still keeping small overall dimensions.